1. Field of the Invention
The invention relates to a method of fabricating a printed wiring board, and more particularly to a method of a fabricating a printed wiring board which method is capable of forming fine through-holes suitable particularly for a highly densified wire arrangement. The invention relates also to a light scattering layer used in the method.
2. Description of the Related Art
A conventional method of fabricating a printed wiring board includes the steps of forming holes with a printed wiring board, plating the holes, applying a photoresist film by electrodeposition coating to a surface of the printed wiring board and inner surfaces of the holes, attaching a mask film to the printed wiring board, placing the printed wiring board on an exposure table, and then radiating ultraviolet Rays to the printed wiring board to thereby expose the photoresist film in selected regions to the ultraviolet Rays. Then, the photoresist film is developed and etched. Thus, there is completed a printed wiring board including desired through-holes, non-desired through-holes and desired circuits. Hereinafter, the term "desired through-holes" mean through-holes having inner surfaces plated with metal such as copper for electrically connecting upper and lower wiring layers with each other therethrough, and the term "non-desired through-holes" mean through-holes having inner surfaces which do not need to be plated with metal, because they are not expected to electrically connect upper and lower wiring layers with each other therethrough. The photoresist film formed by electrodeposition coating may be negative type where a photoresist film is cured when exposed to a light or positive type where a photoresist film is decomposed when exposed to a light.
When desired through-holes and non-desired through-holes are concurrently formed in a printed wiring board, the following steps are carried out.
When negative type photoresist film is employed, a photoresist film coated on inner surfaces of desired through-holes is exposed to ultraviolet rays so as to act as an etching resist. In contrast, non-desired through-holes which are not needed to be plated with copper are not exposed to ultraviolet rays by a mask film covering the non-desired through-holes therewith, and hence a photoresist film coated on inner surfaces of the non-desired through-holes is removed by development. Then, copper is removed in selected regions by etching to thereby form desired through-holes and non-desired through-holes.
When positive type photoresist film is employed, a photoresist film coated on inner surfaces of non-desired through-holes is exposed to ultraviolet rays. Since the photoresist film is optically decomposed by ultraviolet rays, the photoresist film is removed when developed. A photoresist film coated on inner surfaces of desired through-holes which need to be plated with copper is kept from being exposed to ultraviolet rays by a mask film so as to act as an etching resist. Thereafter, copper is removed in selected regions by etching to thereby form desired through-holes and non-desired through-holes.
These days, a printed wiring board is required to have a higher density in wire arrangement. Hence, it is indispensable for a printed wiring board to include fine circuits, desired through-holes having a smaller diameter, but having a higher aspect ratio defined as a ratio of a thickness of a printed wiring board divided by a diameter of a through-hole, and non-desired through-holes having a higher aspect ratio. To this end, as the aspect ratio is got higher and higher, it is necessary for a photoresist film coated on inner surfaces of desired and non-desired through-holes, in particular, having a smaller diameter, to be sufficiently exposed to ultraviolet rays, whichever type of a photoresist film between negative or positive type is to be employed. However, conventional methods of fabricating a printed wiring board have a difficulty in accomplishing the above mentioned matter.
In order to solve such a problem as mentioned above in a method of fabricating a printed wiring board where a photoresist film is deposited by electrodeposition coating, ultraviolet ray scattering has been suggested as a solution to the problem. For instance, Japanese Unexamined Patent Publication No. 4-56189 has suggested a method of scattering parallel beams in order that a photoresist film coated on inner surfaces of holes is exposed to ultraviolet rays to much degree. In the suggested method, there is used a transparent sheet having fine irregularities on a surface thereof.
FIG. 1 is a partial cross-sectional view of a printed wiring board, illustrating the suggested method. A photoresist film 3 is applied by electrodeposition coating onto upper and lower surfaces of a printed wiring board 1 and inner surfaces of desired through-holes 2a and non-desired through-holes 2b formed in the printed wiring board 1 throughout a thickness thereof. The photoresist film 3 is of negative type.
The printed wiring board 1 is covered on upper and lower surfaces thereof with mask films 4 including light-transmissive portions and light-intransmissive portions. In FIG. 1, the light-intransmissive portions are designated as hatched portions, and the light-transmissive portions are designated as non-hatched portions. As illustrated, the light-transmissive portion is in alignment with the desired through-hole 2a, and the light-intransmissive portion is in alignment with the non-desired through-hole 2b.
Light scattering layers 10 comprising a transparent layer having fine irregularities are applied onto the mask films 4 above the upper and lower surfaces of the printed wiring board 1. Transparent frames 5 are applied onto the light scattering layers 10.
Ultraviolet ray sources 6 disposed at opposite sides of the printed wiring board 1 radiate ultraviolet rays 7 onto the printed wiring board 1. The thus radiated ultraviolet rays 7 are refracted in the light scattering layer 10 to thereby establish scattered rays 9a. Since a portion of the photoresist film 3 coated on inner surfaces of the desired through-hole 2a is located below the light-transmissive portion, or non-hatched portion, of the mask film 4, the portion is exposed to the scattered light 9a. Thus, the portion is cured and hence acts as an etching resist. Since a portion of the photoresist film 3 coated on inner surfaces of the non-desired through-hole 2b is located below the light-intransmissive portion, or hatched portion, of the mask film 4, the portion is not exposed to the scattered rays 9a. Thus, the portion is removed in development of the photoresist film. Thereafter, the printed wiring board 1 is etched with the cured photoresist film acting as an etching mask, to thereby form desired and non-desired through-holes.
As mentioned above, the ultraviolet rays 7 transmitted from the ultraviolet ray sources 6 are changed into the scattered lights 9a by the light scattering layers 10. Hence, the portion of the photoresist film 3 coated on inner surfaces of the desired through-holes 2a is exposed to the ultraviolet ray to much more degree, which is effective to desired and non-desired through-holes having a higher aspect ratio, required for highly densified wire arrangement.
However, in accordance with the analysis conducted by the inventor, it has been found that all the parallel ultraviolet rays 7 are not always converted into the scattered rays 9a, and that a part of the parallel ultraviolet rays 7 passes through the mask film 4 without being converted into the scattered light 9a. Thus, the method suggested in Japanese Unexamined Patent Publication No. 4-56189 has an upper limitation for exposing a photoresist film coated on inner surfaces of small holes to ultraviolet rays.